1. Field of the Invention
The invention relates to a valve characteristic control apparatus for an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine, exhaust gas components are purified by a catalyst. However, when a temperature of the catalyst is low when the engine is warming up, or the like, the catalyst is not able to provide adequate purification performance. To address this difficulty, various types of catalyst warming up control are performed in order to rapidly increase the temperature of the catalyst.
For example, in an in-cylinder injection internal combustion engine that can directly inject and supply fuel to the combustion chamber, it is possible to increase a temperature of a combustion gas by, for example, (i) retarding an ignition timing, (ii) increasing an intake air amount to make an air fuel ratio leaner, and (iii) performing fuel injection in the latter half of the compression stroke. Accordingly, in this type of internal combustion engine, a fuel injection timing is set to be in the latter half of the compression stroke during engine warming up. As a result, an exhaust gas temperature increases, whereby the temperature of the catalyst increases rapidly.
At the same time, various types of control are performed to reduce a discharge amount of hydrocarbon (HC) exhausted from the combustion chamber to an exhaust passage during engine warming up. In a device disclosed in Japanese Patent Laid-open Publication No. 2003-120348, for example, a closing timing of an exhaust valve is set to a retard side during engine warming up in order to increase a valve overlap amount. As a result, exhaust gas discharged to an exhaust passage is intaken to the combustion chamber again. Unburned HC contained within the intaken exhaust gas are then combusted again in the following combustion stroke, whereby the HC discharge amount is reduced.
However, in an internal combustion engine in which catalyst warming up control is performed in the manner described above, if a fuel injection amount injected in the latter half of the compression stroke increases excessively, the air fuel ratio in the vicinity of the spark plug becomes excessively rich, which causes the combustion condition of the air fuel mixture to deteriorate. In this case, as a countermeasure, the fuel injection can be separated and performed as a plurality of separated injections. This makes it possible to form a suitable air fuel ratio in the vicinity of the spark plug. However, if any one of the fuel injection amounts injected in the separated injections is less than a minimum injection amount of a fuel injection value (the minimum amount to which the fuel injection amount can be controlled), the separated injections cannot be performed. Thus, non-separated injection is carried out and all of the fuel is injected in the latter half of the compression stroke. Note that, the fuel injection amount is set based on engine load, engine rotational speed, or the like. When the fuel injection amount is not large enough to cause a rich air fuel ratio to be formed in the vicinity of the spark plug, non-separated injection is performed.
However, in an internal combustion engine like that described above, namely, in an internal combustion engine in which the number of times that fuel is injected during one engine cycle (the series of strokes including the intake stroke, compression stroke, combustion stroke and exhaust stroke) is changed, the combustion condition of the air fuel mixture changes depending on the number of injections. Therefore, when retard control of the closing timing of the exhaust valve is performed as described above, the retard amount that is optimal is different depending on the number of injections. However, with the above known retard control, the retard amount is not changed in accordance with the number of injections. As a result, there is need for further improvement in the retard control that is performed for the closing timing of the exhaust value in the above described internal combustion engine.